Lubricating base oils, which comprise a mineral oil contained in petroleum or a synthetic oil such as an ethylene/.alpha.-olefin copolymer, have been hitherto known. Further, lubricating oil compositions containing a viscosity index improver or a lubricating oil additive each comprising an .alpha.-olefin polymer are also known.
The lubricating base oils are desired to be excellent not only in viscosity properties (particularly low-temperature fluidity) but also in heat stability, oxidation stability and lubricating properties.
The lubricating base oils are blended with additives such as an extreme pressure agent, but the lubricating base oils, particularly hydrocarbon synthetic oils, usually have poor compatibility with the lubricating oil additives, and therefore the kind and the amount of the viscosity index improver or the lubricating oil additives added to the lubricating oils have been markedly limited.
Lubricating oil compositions wherein esters are added as compatibility improvers to improve the compatibility of the lubricating base oils with the lubricating oil additives are known.
Even the lubricating oil compositions containing the viscosity index improver or the compatibility improver are desired to be further improved in the heat stability, oxidation stability, lubricating properties and compatibility with the lubricating oil additives. Further, lubricating base oils, which show sufficient compatibility with the lubricating oil additives even if no compatibility improver is used, are desired.
By the way, when middle fraction fuel oils such as gas oil (light oil) and heavy oil are used or stored at low temperatures, crystals of waxes contained in the oils grow. If oils containing grown wax crystals are used, problems such as plugging (clogging) of pipes and extreme increase of viscosity of the whole oil to cause difficulty of oil flow are brought about. To cope with the problems, addition of a fluidity improver such as an ethylene/.alpha.-olefin copolymer to the middle fraction fuel oils is known.
It is also known to add polybutadiene having hydroxyl end group and containing not less than 70% of 1,2-bonds or its hydrogenated product to the middle fraction fuel oils to lower a cold filter plugging point (CFPP).
For purifying automobile exhaust gas, a low-sulfur gas oil, that is a diesel gas oil having a decreased sulfur content, tends to be used, and legal regulation is becoming severe. However, sulfur in the fuel oil contributes to lubricating properties of the fuel oil, and if the sulfur content in the fuel oil is decreased, the lubricating properties of the fuel oil are sometimes lowered to thereby cause abrasion of fuel injection nozzles. Accordingly, fuel oil fluidity improvers having excellent lubricating properties are desired.
The present invention has been made under such circumstances as mentioned above, and it is an object of the invention to provide a lubricating oil and a lubricating oil composition each of which is excellent not only in compatibility with lubricating oil additives but also in viscosity properties, heat stability, oxidation stability and lubricating properties.
It is another object of the invention to provide a lubricating oil composition having excellent properties, which comprises a base oil composed of a mineral oil and/or a hydrocarbon synthetic oil and a viscosity index improver comprising a specific .alpha.-olefin/aromatic vinyl compound random copolymer.
It is a further object of the invention to provide a lubricating oil composition having excellent properties, which comprises a base oil composed of a mineral oil and/or a hydrocarbon synthetic oil, a compatibility improver comprising a specific low-molecular weight .alpha.-olefin/aromatic vinyl compound random copolymer and a lubricating oil additive.
It is a still further object of the invention to provide a viscosity index improver comprising a specific .alpha.-olefin/aromatic vinyl compound random copolymer, and a lubricating oil compatibility improver comprising a specific low-molecular weight .alpha.-olefin/aromatic vinyl compound random copolymer.